This invention relates to machines, such as screen printing machines and dryers, in which light is used to quickly dry a wet surface, e.g., where a multi-color printing process is employed. More specifically, the invention relates to a cover for an ultraviolet (UV) light source.
It is well-known in the printing art that when more than one print image is to be laid upon the same surface, it is necessary to dry the first image before carrying out the second printing step, to avoid smearing and other problems. One method which may be used with some print media is to flash-dry the image by exposure to UV light. Typically, the article subject to printing is moved from a printing station to a UV drying station and then onward to the next printing station. Alternatively, a separate UV drying apparatus may be used in conjunction with a printing machine.
It is the nature of UV light sources that they require a significant period of time to warm up; thus, it is not possible to simply turn them on when an article is ready for UV exposure and then turn them off. A UV light source must remain on if high-speed efficiency is to be achieved in the printing process.
When an article is being exposed to the UV light, the article blocks the light and thereby protects nearby people and objects from exposure; at other times the light would flood the room if left uncovered. UV light, however, can be dangerous for workers who may be subject to exposure. Prolonged exposure can cause skin burns, skin cancer and optical damage. UV light may also bleach or otherwise damage inanimate objects. Consequently, it is important to cover the UV light source during the periods when an article is not directly beneath the UV lamp.
One method for covering the UV lamp is to use a retractable hood. The hood is retracted into an open position for exposure of the article and is closed at other times. These hoods are not adapted for gradual or timed exposure; they have only two positions, open or closed, and are mechanically complicated and thereby expensive to manufacture and maintain. Another disadvantage of these hoods results from the high heat output of UV lamps; because of the heat, rising air currents are produced which draw large quantities of dust and dirt up into the hood. As a result, the hoods require frequent cleaning, perhaps as often as several times a day; unfortunately, the complicated hood mechanism makes cleaning difficult and time-consuming.